VSCF systems have been utilized to convert variable frequency electrical power developed by a generator driven by a variable speed prime mover into constant frequency power for AC loads. Such systems include a rectifier for rectifying the variable frequency output of the generator and an inverter for converting the rectified output into the constant frequency power. In prior VSCF systems, the inverter includes a number of bipolar switching transistors which are controlled by base drive circuitry. Also, an input autotransformer is connected between the generator output and the rectifier input to permit high voltage AC generation and distribution to the inverter which in turn reduces the weight of feeders coupled to the inverter.
It has been found that the cost of bipolar transistors and the required base drive circuits is relatively high. It has also been found that the weight of the input autotransformer together with the weight of a filter coupled to the inverter output cause the overall system weight to be undesirably increased.
The costs associated with the inverter can be reduced by utilizing other types of switching devices, such as insulted gate bipolar transistors (IGBT's), power FET's or MOS controlled thyristors (MCT's). Such types of switches are more cost effective than the bipolar transistor; however, these devices do not have the high current handling capabilities of bipolar transistors. Therefore, if such devices are to be used, they must be operated in parallel. As is generally known, operating semiconductor switching devices in parallel presents significant difficulties. Specifically, the characteristics of these devices must be carefully matched to insure that the currents handled by the devices are shared as equally as possible so that the potential of damage or destruction of the switches is minimized.
Inverters have been designed which include parallel power switches that are connected to end terminals of an interphase transformer having a mid-tap at which an output is produced. The interphase transformer is designed so that the output voltage at the mid-tap is at a level intermediate the voltages applied by the switches. In this type of inverter, each switch handles only a portion of the total output current magnitude. Patents disclosing such inverters include Urish, U.S. Pat. No. 3,657,633, Heintze, U.S. Pat. No. 3,943,429, Honbu, et al., U.S. Pat. No. 4,549,258 and Mizoguchi, U.S. Pat. No. 4,802,079.
Shilling, et al., U.S. Pat. No. 4,743,777 discloses a VSCF starter generator system using a brushless, synchronous generator. When operating in a starting mode at the beginning of a starting sequence, AC power developed by the external AC power source is directly applied to the three-phase AC exciter field windings. The AC power developed by the external AC source is further provided to a variable voltage, variable frequency power converter which in turn provides a controlled voltage and frequency to armature windings of a main generator. The AC power provided to the AC exciter field windings is transferred by transformer action to exciter armature windings disposed on a rotor of the generator. This AC power is rectified by a rotating rectifier and provided to a main field winding of the generator. The interaction of the magnetic fields developed by the main generator field winding and armature windings in turn causes the rotor of the generator to rotate and thereby develop the desired motive power. When the generator is operated in a generating mode, switches are operated to disconnect the AC exciter field windings and the main generator armature windings from the external AC source and to connect the power converter to the main generator armature windings. The power converter is thereafter operated to convert the variable frequency output of the generator into the designed fixed frequency power.
Messenger, U.S. Pat. No. 3,908,161 discloses a brushless generator including three exciter field windings which are connected in a wye configuration and provided three-phase AC power during operation in a starting mode. The three-phase AC power induces AC power in an exciter armature winding which is rectified and applied to a main generator field winding. Main armature windings receive controlled AC power to in turn cause rotation of the generator rotor. Thereafter, the three exciter field windings are connected in series and provided DC excitation when operating in a generating mode.